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Magnetic anisotropy in obliquely deposited cobalt films
Journal of Magnetism and Magnetic Materials 148 (1995) 132-133
anisotropy in obliquely deposited cobalt films K.
Itoh
a9*, K. Hara a, M. Kamiya a, K. Bkamoto b, T. Hashimoto ‘, H. Fujiwara d a Faculty of General Education, Kumamoto University, Kumamoto 860, Japan b Faculty of EducatioG Chiba Universi@, Chiba 263, Japan ’ Faculty of Education, Tottori University, Tottcri 680, Japan ’ Faculty of Literature, Hiroshima Jogakuin University, Hiroshima 732, Japan
Abstract The anisotropy field in cobalt films deposited obliquely by sputtering was separated into the magnetocrystalline anisotropy field, H,(crysO, and the anisotropy field due to the shape anisotropy, H,(shape), based on the (00021 pole figure. The incidence angle was 45” and the substrate temperature was 332 K. The film thickness ranged from 0.4 to 3.8 pm- The thickness dependence of H&hape) is similar to that of the anisotropy of the reflection coefficient indicating that our separation of Hk is right on the whole.
I. Introduction In cobalt films deposited obliquely by sputtering we found a new type of columnar grain structure, in which the columnar grains align in the direction parallel to the incidence pIane. This alignment can not be explained by the geometric shadowing mechanism [ll or by the inhibited mobility of adatoms [2], so we must consider another mechanism. In order to investigate the formation mechanism of the columnar grain structure the magnetic analysis is effective because the columnar grain structure induces the magnetic anisotropy through the shape anisotropy. In cobalt films a magnetic anisotropy is caused by the magnetocrystalline anisotropy due to the c-axis orientation as well as the shape anisotropy of the columnar grain structure [3]. Therefore, in order to investigate magnetically the columnar grain structure of cobalt films it is necessary to separate the magnetic anisotropy into the shape and crystalline components. WC have reported [4] that the contribution of the magnetocrystalline anisotropy in obliquely deposited hexagonal films can be estimated based on the (0002) pole figure. In this work, according to the procedure reported we estimate the magnetocrystalline anisotropy in cobalt films with a new type of columnar grain structure,and extract the magnetic anisotropy due to the shape anisotropy. 2. Experimental Cobalt films were deposited on a glass microscope slide substrate using a conventional rf diode sputtering
* Corresponding author. Fax: + 81-96-345-8907. 0304-8853/95/$09.50
apparatus at a power of 100 W and at an argon gas pressure of 20 Pa. The incidence angle defined as the angle between the substrate normal and the target normal was 45”. The substrate temperature was 332 K. The deposition rate was 0.5 nm s-l and the film thickness ranged from 0.4 to 3.8 km. The determination of the magnetic anisotropy was made by the torque measurement in the film plane and the magnetization measurement. The alignment of crystallites was quantitatively analysed using reflection ellipsometry [5]. The light source was a He-Ne gas laser and the wavelength was 633 nm. The (0002] pole densities were determined using the X-ray Schulz method [6]. The radiation used was Fe Ka. 3. Results
Fig, l(a) shows the magnetic anisotropy field Hk of the sputtered cobalt films as a function of the film thickness. The field Nk is positive in the large thickness range (> 0.8 pm) and increases with increasing thickness. For the purpose of estimating the shape anisotropy of columnar grains, we have ineasured the anisotropy of the reflection coefficient using reflection ellipsometry. Fig. l(b) shows the thickness dependence of the anisotropy of reflection coefficient, Ar/F for the same films referred to in Fig. l(a), where Ar is the difference between the coefficients along the directions perpendicular and paraitel to the incidence plane and P is the average value of the coefficients. The anisotropy, Ar/F, is negative and its magnitude increases with increasing thickness. The negative sign of Ar/? means the columnar grains are aligned in the direa tion parallel to the incidence plane. This alignment contributes negatively to N,. The positive Hk and negative Ar/F suggest that there is positive magnetic anisotropy
0 1995 Elsevier Science B.V. All rights reserved
SSDZ 0304~8853(95)00178-6
and discussion
K. Itoha et al./Joumal
1_I.
of Magnetism and Magnetic Materials 148 (199.5) 132-133
155
(a) 6 0
t 5 z u? -z= P
3
.G 1
2
P film lhickness
“H 4
0
D
1 0 l/i.-.,
,
,
,
(pm) 0
1
2
3
4
fi\m thickness (pm) film thickness
0.0 1L
-0.2
2 z
-0.4
-0.6
0
(Mm)
I
2
3
4
1
I
I
l
T-x--
Fig. 1. The thickness dependence of (a) HL and (b) Ar/F
03
film thickness (pm1 0
0
1
2
3
4
,
I
I
I
I
for
sputtered cobalt films.
Fig. 3. The thickness
dependence
of (a) H&ryst)
and
H,(shape). due to the c-axis orientation
whose magnitude exceeds that of the observed anisotropy. Fig. 2 shows the (0002) perspective pole figure of the film 3 p.m in thickness. The polar net is in the ;ry plane. The center of the polar net is the film normal and the beam direction lies on the y-axis. The z-coordinate expresses the pole density. As seen in the figure, the (0002) pole localizes in the direction perpendicular to the incidence plane. This c-axis orientation induces a positive in-plane anisotropy field. By using the distribution function of
(0002) poles [4], we estimated the in-plane anisotropy field due to the c-axis orientation, H&ryst). The thickness dependence of H,(cryst) is shown in Pig. 3(a). The field, H&yst), is positive and increases with increasing thickness. & was suggested, H&q&) > H, in all the films. The in-plane anisotropy field due to the shape anisotropy, H,(shape), was evaluated by the relation H,(shape)
= Hk - H,(cryst).
(1)
In Fig. 3(b) H,(shape) is plotted as a function of the film thickness. The field, H&shape), is negative and its magnitude increases with increasing film thickness. The thickness dependence of H&hape) is similar to that of Ar/F shown in Fig. l(b). This fact indicates that our separation of Hk is right on the whole. References
Fig. 2. The {OOOZ}perspective pole figure of the film 3 &rn in thickness.
[l] D.O. Smith, MS. Cohen and G.P. Weiss, J. Appl. Phys. 31 (1960) 1755. [2] J.G.W. van de Waterbeemd and G.W. van Oosterhout, Philips Res. Rep. 22 (1967) 375. [3] K. Itoh, J. Magn. Magn. Mater. 95 (1991) 237. [4] K. Okamoto, K. Itoh and T. Hashimoto, J. Maga. Magn. Mater, 87 (1990) 379. [S] M. Kamiya, K. Ham, T. Hashimoto and H. Pujiwara, J. Phys. Sot. Jpn. 52 (1983) 3585; 53 (1984) 468 bxraiumi. [6] LG. Schulz, 3. AppI. Phys. 20 (1949) 1030.
anisotropy in obliquely deposited cobalt films K.
Itoh
a9*, K. Hara a, M. Kamiya a, K. Bkamoto b, T. Hashimoto ‘, H. Fujiwara d a Faculty of General Education, Kumamoto University, Kumamoto 860, Japan b Faculty of EducatioG Chiba Universi@, Chiba 263, Japan ’ Faculty of Education, Tottori University, Tottcri 680, Japan ’ Faculty of Literature, Hiroshima Jogakuin University, Hiroshima 732, Japan
Abstract The anisotropy field in cobalt films deposited obliquely by sputtering was separated into the magnetocrystalline anisotropy field, H,(crysO, and the anisotropy field due to the shape anisotropy, H,(shape), based on the (00021 pole figure. The incidence angle was 45” and the substrate temperature was 332 K. The film thickness ranged from 0.4 to 3.8 pm- The thickness dependence of H&hape) is similar to that of the anisotropy of the reflection coefficient indicating that our separation of Hk is right on the whole.
I. Introduction In cobalt films deposited obliquely by sputtering we found a new type of columnar grain structure, in which the columnar grains align in the direction parallel to the incidence pIane. This alignment can not be explained by the geometric shadowing mechanism [ll or by the inhibited mobility of adatoms [2], so we must consider another mechanism. In order to investigate the formation mechanism of the columnar grain structure the magnetic analysis is effective because the columnar grain structure induces the magnetic anisotropy through the shape anisotropy. In cobalt films a magnetic anisotropy is caused by the magnetocrystalline anisotropy due to the c-axis orientation as well as the shape anisotropy of the columnar grain structure [3]. Therefore, in order to investigate magnetically the columnar grain structure of cobalt films it is necessary to separate the magnetic anisotropy into the shape and crystalline components. WC have reported [4] that the contribution of the magnetocrystalline anisotropy in obliquely deposited hexagonal films can be estimated based on the (0002) pole figure. In this work, according to the procedure reported we estimate the magnetocrystalline anisotropy in cobalt films with a new type of columnar grain structure,and extract the magnetic anisotropy due to the shape anisotropy. 2. Experimental Cobalt films were deposited on a glass microscope slide substrate using a conventional rf diode sputtering
* Corresponding author. Fax: + 81-96-345-8907. 0304-8853/95/$09.50
apparatus at a power of 100 W and at an argon gas pressure of 20 Pa. The incidence angle defined as the angle between the substrate normal and the target normal was 45”. The substrate temperature was 332 K. The deposition rate was 0.5 nm s-l and the film thickness ranged from 0.4 to 3.8 km. The determination of the magnetic anisotropy was made by the torque measurement in the film plane and the magnetization measurement. The alignment of crystallites was quantitatively analysed using reflection ellipsometry [5]. The light source was a He-Ne gas laser and the wavelength was 633 nm. The (0002] pole densities were determined using the X-ray Schulz method [6]. The radiation used was Fe Ka. 3. Results
Fig, l(a) shows the magnetic anisotropy field Hk of the sputtered cobalt films as a function of the film thickness. The field Nk is positive in the large thickness range (> 0.8 pm) and increases with increasing thickness. For the purpose of estimating the shape anisotropy of columnar grains, we have ineasured the anisotropy of the reflection coefficient using reflection ellipsometry. Fig. l(b) shows the thickness dependence of the anisotropy of reflection coefficient, Ar/F for the same films referred to in Fig. l(a), where Ar is the difference between the coefficients along the directions perpendicular and paraitel to the incidence plane and P is the average value of the coefficients. The anisotropy, Ar/F, is negative and its magnitude increases with increasing thickness. The negative sign of Ar/? means the columnar grains are aligned in the direa tion parallel to the incidence plane. This alignment contributes negatively to N,. The positive Hk and negative Ar/F suggest that there is positive magnetic anisotropy
0 1995 Elsevier Science B.V. All rights reserved
SSDZ 0304~8853(95)00178-6
and discussion
K. Itoha et al./Joumal
1_I.
of Magnetism and Magnetic Materials 148 (199.5) 132-133
155
(a) 6 0
t 5 z u? -z= P
3
.G 1
2
P film lhickness
“H 4
0
D
1 0 l/i.-.,
,
,
,
(pm) 0
1
2
3
4
fi\m thickness (pm) film thickness
0.0 1L
-0.2
2 z
-0.4
-0.6
0
(Mm)
I
2
3
4
1
I
I
l
T-x--
Fig. 1. The thickness dependence of (a) HL and (b) Ar/F
03
film thickness (pm1 0
0
1
2
3
4
,
I
I
I
I
for
sputtered cobalt films.
Fig. 3. The thickness
dependence
of (a) H&ryst)
and
H,(shape). due to the c-axis orientation
whose magnitude exceeds that of the observed anisotropy. Fig. 2 shows the (0002) perspective pole figure of the film 3 p.m in thickness. The polar net is in the ;ry plane. The center of the polar net is the film normal and the beam direction lies on the y-axis. The z-coordinate expresses the pole density. As seen in the figure, the (0002) pole localizes in the direction perpendicular to the incidence plane. This c-axis orientation induces a positive in-plane anisotropy field. By using the distribution function of
(0002) poles [4], we estimated the in-plane anisotropy field due to the c-axis orientation, H&ryst). The thickness dependence of H,(cryst) is shown in Pig. 3(a). The field, H&yst), is positive and increases with increasing thickness. & was suggested, H&q&) > H, in all the films. The in-plane anisotropy field due to the shape anisotropy, H,(shape), was evaluated by the relation H,(shape)
= Hk - H,(cryst).
(1)
In Fig. 3(b) H,(shape) is plotted as a function of the film thickness. The field, H&shape), is negative and its magnitude increases with increasing film thickness. The thickness dependence of H&hape) is similar to that of Ar/F shown in Fig. l(b). This fact indicates that our separation of Hk is right on the whole. References
Fig. 2. The {OOOZ}perspective pole figure of the film 3 &rn in thickness.
[l] D.O. Smith, MS. Cohen and G.P. Weiss, J. Appl. Phys. 31 (1960) 1755. [2] J.G.W. van de Waterbeemd and G.W. van Oosterhout, Philips Res. Rep. 22 (1967) 375. [3] K. Itoh, J. Magn. Magn. Mater. 95 (1991) 237. [4] K. Okamoto, K. Itoh and T. Hashimoto, J. Maga. Magn. Mater, 87 (1990) 379. [S] M. Kamiya, K. Ham, T. Hashimoto and H. Pujiwara, J. Phys. Sot. Jpn. 52 (1983) 3585; 53 (1984) 468 bxraiumi. [6] LG. Schulz, 3. AppI. Phys. 20 (1949) 1030.